package com.clark.io;

import java.io.File;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;

/**
 * Keeps track of files awaiting deletion, and deletes them when an associated
 * marker object is reclaimed by the garbage collector.
 * <p>
 * This utility creates a background thread to handle file deletion. Each file
 * to be deleted is registered with a handler object. When the handler object is
 * garbage collected, the file is deleted.
 * <p>
 * In an environment with multiple class loaders (a servlet container, for
 * example), you should consider stopping the background thread if it is no
 * longer needed. This is done by invoking the method {@link #exitWhenFinished},
 * typically in {@link javax.servlet.ServletContextListener#contextDestroyed} or
 * similar.
 * 
 * @author Noel Bergman
 * @author Martin Cooper
 * @version $Id: FileCleaningTracker.java 1005707 2010-10-08 04:02:57Z niallp $
 */
public class FileCleaningTracker {
    /**
     * Queue of <code>Tracker</code> instances being watched.
     */
    ReferenceQueue<Object> q = new ReferenceQueue<Object>();
    /**
     * Collection of <code>Tracker</code> instances in existence.
     */
    final Collection<Tracker> trackers = Collections
            .synchronizedSet(new HashSet<Tracker>()); // synchronized
    /**
     * Collection of File paths that failed to delete.
     */
    final List<String> deleteFailures = Collections
            .synchronizedList(new ArrayList<String>());
    /**
     * Whether to terminate the thread when the tracking is complete.
     */
    volatile boolean exitWhenFinished = false;
    /**
     * The thread that will clean up registered files.
     */
    Thread reaper;

    // -----------------------------------------------------------------------
    /**
     * Track the specified file, using the provided marker, deleting the file
     * when the marker instance is garbage collected. The
     * {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
     * 
     * @param file
     *            the file to be tracked, not null
     * @param marker
     *            the marker object used to track the file, not null
     * @throws NullPointerException
     *             if the file is null
     */
    public void track(File file, Object marker) {
        track(file, marker, (FileDeleteStrategy) null);
    }

    /**
     * Track the specified file, using the provided marker, deleting the file
     * when the marker instance is garbage collected. The speified deletion
     * strategy is used.
     * 
     * @param file
     *            the file to be tracked, not null
     * @param marker
     *            the marker object used to track the file, not null
     * @param deleteStrategy
     *            the strategy to delete the file, null means normal
     * @throws NullPointerException
     *             if the file is null
     */
    public void track(File file, Object marker,
            FileDeleteStrategy deleteStrategy) {
        if (file == null) {
            throw new NullPointerException("The file must not be null");
        }
        addTracker(file.getPath(), marker, deleteStrategy);
    }

    /**
     * Track the specified file, using the provided marker, deleting the file
     * when the marker instance is garbage collected. The
     * {@link FileDeleteStrategy#NORMAL normal} deletion strategy will be used.
     * 
     * @param path
     *            the full path to the file to be tracked, not null
     * @param marker
     *            the marker object used to track the file, not null
     * @throws NullPointerException
     *             if the path is null
     */
    public void track(String path, Object marker) {
        track(path, marker, (FileDeleteStrategy) null);
    }

    /**
     * Track the specified file, using the provided marker, deleting the file
     * when the marker instance is garbage collected. The speified deletion
     * strategy is used.
     * 
     * @param path
     *            the full path to the file to be tracked, not null
     * @param marker
     *            the marker object used to track the file, not null
     * @param deleteStrategy
     *            the strategy to delete the file, null means normal
     * @throws NullPointerException
     *             if the path is null
     */
    public void track(String path, Object marker,
            FileDeleteStrategy deleteStrategy) {
        if (path == null) {
            throw new NullPointerException("The path must not be null");
        }
        addTracker(path, marker, deleteStrategy);
    }

    /**
     * Adds a tracker to the list of trackers.
     * 
     * @param path
     *            the full path to the file to be tracked, not null
     * @param marker
     *            the marker object used to track the file, not null
     * @param deleteStrategy
     *            the strategy to delete the file, null means normal
     */
    private synchronized void addTracker(String path, Object marker,
            FileDeleteStrategy deleteStrategy) {
        // synchronized block protects reaper
        if (exitWhenFinished) {
            throw new IllegalStateException(
                    "No new trackers can be added once exitWhenFinished() is called");
        }
        if (reaper == null) {
            reaper = new Reaper();
            reaper.start();
        }
        trackers.add(new Tracker(path, deleteStrategy, marker, q));
    }

    // -----------------------------------------------------------------------
    /**
     * Retrieve the number of files currently being tracked, and therefore
     * awaiting deletion.
     * 
     * @return the number of files being tracked
     */
    public int getTrackCount() {
        return trackers.size();
    }

    /**
     * Return the file paths that failed to delete.
     * 
     * @return the file paths that failed to delete
     * @since Commons IO 2.0
     */
    public List<String> getDeleteFailures() {
        return deleteFailures;
    }

    /**
     * Call this method to cause the file cleaner thread to terminate when there
     * are no more objects being tracked for deletion.
     * <p>
     * In a simple environment, you don't need this method as the file cleaner
     * thread will simply exit when the JVM exits. In a more complex
     * environment, with multiple class loaders (such as an application server),
     * you should be aware that the file cleaner thread will continue running
     * even if the class loader it was started from terminates. This can
     * consitute a memory leak.
     * <p>
     * For example, suppose that you have developed a web application, which
     * contains the commons-io jar file in your WEB-INF/lib directory. In other
     * words, the FileCleaner class is loaded through the class loader of your
     * web application. If the web application is terminated, but the servlet
     * container is still running, then the file cleaner thread will still
     * exist, posing a memory leak.
     * <p>
     * This method allows the thread to be terminated. Simply call this method
     * in the resource cleanup code, such as
     * {@link javax.servlet.ServletContextListener#contextDestroyed}. Once
     * called, no new objects can be tracked by the file cleaner.
     */
    public synchronized void exitWhenFinished() {
        // synchronized block protects reaper
        exitWhenFinished = true;
        if (reaper != null) {
            synchronized (reaper) {
                reaper.interrupt();
            }
        }
    }

    // -----------------------------------------------------------------------
    /**
     * The reaper thread.
     */
    private final class Reaper extends Thread {
        /** Construct a new Reaper */
        Reaper() {
            super("File Reaper");
            setPriority(Thread.MAX_PRIORITY);
            setDaemon(true);
        }

        /**
         * Run the reaper thread that will delete files as their associated
         * marker objects are reclaimed by the garbage collector.
         */
        @Override
        public void run() {
            // thread exits when exitWhenFinished is true and there are no more
            // tracked objects
            while (exitWhenFinished == false || trackers.size() > 0) {
                try {
                    // Wait for a tracker to remove.
                    Tracker tracker = (Tracker) q.remove(); // cannot return
                    // null
                    trackers.remove(tracker);
                    if (!tracker.delete()) {
                        deleteFailures.add(tracker.getPath());
                    }
                    tracker.clear();
                } catch (InterruptedException e) {
                    continue;
                }
            }
        }
    }

    // -----------------------------------------------------------------------
    /**
     * Inner class which acts as the reference for a file pending deletion.
     */
    private static final class Tracker extends PhantomReference<Object> {

        /**
         * The full path to the file being tracked.
         */
        private final String path;
        /**
         * The strategy for deleting files.
         */
        private final FileDeleteStrategy deleteStrategy;

        /**
         * Constructs an instance of this class from the supplied parameters.
         * 
         * @param path
         *            the full path to the file to be tracked, not null
         * @param deleteStrategy
         *            the strategy to delete the file, null means normal
         * @param marker
         *            the marker object used to track the file, not null
         * @param queue
         *            the queue on to which the tracker will be pushed, not null
         */
        Tracker(String path, FileDeleteStrategy deleteStrategy, Object marker,
                ReferenceQueue<? super Object> queue) {
            super(marker, queue);
            this.path = path;
            this.deleteStrategy = (deleteStrategy == null ? FileDeleteStrategy.NORMAL
                    : deleteStrategy);
        }

        /**
         * Return the path.
         * 
         * @return the path
         */
        public String getPath() {
            return path;
        }

        /**
         * Deletes the file associated with this tracker instance.
         * 
         * @return <code>true</code> if the file was deleted successfully;
         *         <code>false</code> otherwise.
         */
        public boolean delete() {
            return deleteStrategy.deleteQuietly(new File(path));
        }
    }

}
